Device to Device (D2D) communication, i.e., direct communication between devices, refers to a communication mode in which adjacent User Equipments (UEs) can transmit data on a direct link in a short-distance range without the data being forwarded by a central node (i.e., a base station).
There are generally the following scenarios in the studies on D2D communication, where more than one of the scenarios can occur concurrently:
An out of coverage scenario as illustrated in FIG. 1A where both a UE 1 and a UE 2 reside outside a coverage area;
A partial coverage scenario as illustrated in FIG. 1B where the UE 1 resides in the coverage area of a cell (in coverage), and the UE 2 resides outside the coverage area of the cell;
An in coverage-single-cell scenario as illustrated in FIG. 1C where the UE 1 and the UE 2 reside in the coverage area of the same cell; and
An in coverage-multi-cell scenario as illustrated in FIG. 1D where the UE 1 and the UE 2 reside in the coverage areas of different cells.
Particularly if the UE 1 is a transmitting UE transmitting information, then the UE 2 receives the information transmitted by the UE 1. Alternatively the UE 2 can be a transmitting UE transmitting information, and the UE 1 can receive the information transmitted by the UE 2.
In D2D communication, in addition to the one-to-one communication mode between D2D UEs, there is another application scenario where group/broadcast communication is conducted between the D2D UEs, which can be applicable to fire fighting, rescue, antiterrorism, etc., among public security applications.
The Long Term Evolution (LTE) D2D technology refers to D2D discovery and communication procedures, controlled by an LTE network, operating in an LTE licensed frequency band. On one hand, the advantages of the D2D technology can be fully utilized, and on the other hand, some problems of the traditional D2D technology, e.g., uncontrollable interference, etc., can be overcome due to the control by the LTE network. The introduction of the LTE D2D characteristics enables the LTE technology to evolve from the simple radio mobile cellular communication technology toward the universal connectivity technology.
In LTE D2D transmission, in a contention-based resource allocation method, the respective D2D UEs compete for resources for D2D transmission under the same rule. This method is generally applicable to such a scenario that there is no central scheduling node, e.g., the out of coverage scenario above, where the respective D2D UEs compete randomly for transmission resources without centralized scheduling, thus resulting in resource collision.